Method of geophysical exploration



Jan. 24, 1939. F, RIEBER 2,144,812

METHOD OF GEOPHYSICAL EXPLORATION Filed 001:. 3, 1934 4 Sheets-Sheet l INVENTOR. FRANK E/EBEE HIS A TTORNE YS.

Jan. 24, 1939.

F. RIEBER 2,144,812

METHOD OF GEOPHYSICAL EXPLORATION Filed Oct. 5, 1954 4 Sheets-Sheet 2 INVENTOR. PEA/Y? Q EBEE A WAZQMW HA5 A TTORNEYS.

Jan. 24, 1939. Fv RIEBER 2,144,812

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' UNITED STATES PATENT OFFICE METHOD OF GEOPHYSICAL EXPLORATION Frank Richer, Los Angeles, Calif., assignor of one-half to Continental Oil Company Application October 3,

13 Claims.

This invention relates to methods of geophysical exploration, 1. e., the methods of determining the nature and disposition of the geological structures below the surface of the earth, and particularly to such methods employing elastic waves initiated and transmitted within the earth for imparting the desired information.

The general method of geophysical exploration by means of elastic waves within the earth has long been known. In general this method comprises initiating an impulse somewhere within the earths crust, and recording the resultant earth movement at a point more or less spaced from the point of origin of the impulse, in such manner that the time elapsing between the instant of the impulse and the ensuing movement may be determined. The original impulse sets up an elastic wave which is transmitted through the earth, and this wave, in general, will comprise a damped wave train. Any discontinuity or variation of structure within the earth will reflect and/or retract or diffract this wave train or a portion thereof, so that the record made at the receiving point will comprise a number of arriving waves, each derived from the original impulse, and each differing from the others in time of arrival, magnitude or both. The information desired is derived from the record by determining the instant of each successive arrival.

Where the discontinuities in structure are widely separated and large in magnitude, i. e., where the strata are thick and the differences in density or elasticity as between adjacent strata are large, fairly, satisfactory results have, in the past, been obtainable from this method. But where the strata are thin and the difierences in density and elasticity as between the strata are small, the record will be the result of an extremely large number of wave trains of similar magnitude and random phase. Such a record is of extremely complex character. The determination of the instant of arrival of the various trains can only be approximated by rule of thumb methods wherein the proportional errors are large, and the information to be gained from the record is so unreliable and contradictory as to make the method of little value.

In practice two general modifications of the general method have been used. In the first, usually known as refraction shooting, the earth movements at a point rather widely separated from the point of origin of the impulse are recorded, the information-bearing waves reaching the receiver along refracted paths of such length 55 that those portions of the wave arriving through 1934, Serial No. 746,681

deeper-lying strata reach the receiver before those transmitted wholly through the surface layers, owing to the higher velocity of propagation in the deeper and denser material. This modification is tedious and uneconomical, since a large number of records of high intensity shots must be made to get the required data.

In the second modification, usually known as reflection shooting, the receiving point is relatively close to the wave source, and the waves reach the receiver along reflected paths, arriving seriatim as reflected from the surfaces of successively deeper strata. This modification is more economical of explosives, since the wave paths are shorter and the attenuation of the received waves is correspondingly less, and since more information is obtainable from a single shot. The arrivals are more closely spaced, however, and the confusion of the record is accordingly greater, and owing to the difiiculty of interpretation the method has met with little success in comparison to its theoretical advantages.

In the interpretation of either type of record the general practice is to determine the instant of the peak of the greatest amplitude within the range of the record under immediate examination, and to assume that this peak occurred at a definite time interval after the first arrival of the wave impulse. Since, however, a newly arriving wave may appear on the record as either an increase, or decrease in amplitude, or as a mere change of phase, depending on whether the new arrival is in or out of step with a previous arrival, the errors involved may easily be so great as to destroy wholly the value of the interpretations for any except the simplest structures.

The broad purpose of this invention is to provide a method of determining accurately the instant of arrival of those wave trains from which knowledge of the underlying structure is obtainable, and of analyzing these wave trains to derive therefrom the maximum of accurate information concerning these structures. With these ends in view, the objects of my invention are:

To provide a method of accentuating the recorded initial impulse of an arriving wave and of suppressing the subsequent damped oscillation; to provide a method of combining the information received at a plurality of positions so as to determine accurately the shape of arriving wave fronts, and hence the effective direction of propagation of the arriving wave; to provide a method of suppressing the effect of casual and undifferentiated waves, whose interpretation is impossible and which mask or confuse the information-bearing waves; to provide a method of analysis which will deliver the maximum information in the shortest time, and by routine or semi-automatic procedure; to provide a meth- 0d of recording an analysis of received waves which permits and makes possible the determination of structures comprising closely adjacent strata having relatively small differences in propagation characteristics; to provide a method of receiving and analyzing elastic wave trains which is equally applicable to both reflection and refraction shooting; and, broadly, to provide a method of analysis of complex waves of random phase, including transient and steady components, which permits suppression of undesired components and the accentuation of desired components.

Referring to the drawings:

Figure 1 is a series of graphs showing the nature of an initial wave impulse and the ensuing wave motion, together with conventionalized representations thereof and illustrations of certain components as indicated by analysis as herein described.

Figure 2 is a schematic diagram illustrating the conditions obtaining in geophysical exploration by refraction shooting.

Figure 3 is a conventional representation of the type of record obtained under the conditions shown in Figure 2.

Figure 4 is an illustration of the type of trace obtained upon analysis of the waves received under the conditions shown in Figure 2, when analyzed by the method herein described.

Figure 5 is a diagrammatic illustration of the conditions obtaining in "reflection shooting.

Figures 6 and 7 illustrate records of the received waves and their analysis, as produced under conditions of Figure 5, in the same general manner as Figures 3 and 4. t

Figure 8 isa schematic diagram illustrating a means of initially recording these waves in accordance with the method of this invention.

Figure 9 is a diagrammatic showing of the disposition of the apparatus shown in Figure 8, illustrating the use of the herein described method for determining the shape of a wave front and the eiiective direction of propagation of the received wave.

Figure 10 is a schematic diagram of apparatus for translating and analyzing records obtained with the equipment and methods illustrated in Figures 8 and 9.

Figure 11 illustratesone method of analysis whereby an initial impulse is separated from the ensuing wave trainin accordance with the method of my invention.

' determined angle or direction.

Figure 15 is a graph showing the response curve of the recorder lamp and the aperture distortion curve.

In general terms, the method of my invention comprises recording an arriving wave train or group thereof, and subsequently re-recording a group oi components derived from the original record at diflerent time phases, in such manner that the derived components are combined in a phase relation which suppresses or phases out" undesired components, while accentuating or adding the desired components, and more particularly the transient impulses which initiate the waves. Preferably the method involves making the initial record in phonographic form, i. e., of such a character that it may be translated into electrical currents or mechanical movements, and the record may therefore be made by any of the known processes which are employed in the phonographic recordation and reproduction of sound. Once recorded, the reccm is reproduced or translated from a plurality of points which differ in time phase relative to the record. Most conveniently, electrical reproduction is used, and the currents from the plurality of reproducing pointsare mixed and utilized to operate an analyzer. The analyzer I may conveniently comprise a recording galvanometer, which records the amplitude of the com-' bined components and records them in visible form as a graph or analyzer trace.

' The rationale of this method may more clearly be perceived from the following detailed description, wherein the process of making and analyzing the various records is followed step by step. First, however, it is probably desirable to define what is meant by time phase as used in this specification. The waves here considered have a finite velocity of propagation, and hence their phases will differ at the same instant at different points along any line of propagation. All phases of the wave may exist in the earth at the same time, and, (neglecting attenuation of certain components) all phases may be recorded at a plurality of points, but the same phase will be recorded at each point at a diiferent time. These differences in time are translated into difierences of position along the record. For want of a better term, points on the various records which are recorded at the same instant are referred to as being in the same time phase, for it is not any absolute time that is important, but the times relative to the instant of origin of the wave, or relative to the instant of arrival of any phase at a given point. Hence when a group of records are so reproduced that portions of the tracks made at diflferent instants are reproduced at the same instant, they are referred to as reproduced in different time phase relationship, relative to the record or to each other.

Referring to Figure 2, which illustrates schematically the conditions obtaining in refraction shooting", a charge of explosive I, buried more or less'deeply in the surface layer 2 of the earth, is fired electrically by the use of the usual battery 4. At some distance from the explosive charge there is also embedded in the layer 2 a pickup microphone 5, preferably of the acceleration sensitive type. although any device responsive to elastic waves may be used. Connected with the microphone by the leads 6 is an amplifier and recorder 1, whose characteristics will be considered in detail later, it suflicing for the present to state that it includes suitable means for tracing out the wave form of the impulses transmitted to it by the microphone, and also, preferably, timing means, such as an electrically driven tuning fork for marking time intervals along the record, which latter is indicated in the schematic drawing by the strip 9. Suitable means, not shown, are of course provided for driving the rec- 0rd. Furthermore,'the recorder is connected by u a line ill with the point at which the explosion is to take place. The purpose of this line is to transmit an impulse marking the instant at which the explosion occurs, a simple arrangement being the inclusion within this line of a length of wire which is broken by the explosion, opening the line, the break in the circuit thus occurring being marked upon the record.

When the firing battery 4 is operated and the explosion takes place, there is a sudden release of gas, with a corresponding rise of pressure which crushes the immediately surrounding soil to some extent (i. e., stresses it beyond the elastic limit). Beyond the radius of crushing, the earth is momentarily thrust outward. Being, to some extent at least, an elastic medium, the soil beyond the region wherein crushing occurs has both inertia and a restoring force, and hence its initial excursion is beyond that which it would make were the same pressure applied gradually and steadily, following which it springs back .and finally comes to rest after a damped oscillation.

A general type of movement of the earth is illustrated in graph A of Figure 1, wherein the initial transient or impulse imparted by the explosive is shown by the initial rise H of the curve, while the ensuing damped wave train is shown by the portion i2. It is characteristic of oscillations of this type that the decay of the oscillation is logarithmic, i. e., each succeeding swing is of less amplitude than the preceding excursion by the same ratio.

It will be convenient hereafter to illustrate such wave trains as envelopes, i. e., as curves which are symmetrical about the time axis, one-half of the envelope following the curve of the initial impulse as far as the crest of the first wave, and from this point developing into a smooth curve of logarithmic form tangent to each of the successive crests, positive and negative, of the wave, as illustrated in graph 13 of Figure I, wherein the curve i3 is the envelope while the damped wave which the envelope symbolizes is shown by the dotted curve it.

The damped wave initiated by the explosion is propagated in all directions through'the earth. If the structure were entirely homogeneous the wave would travel, as a spherical or hemispherical wave front, with equal velocity in all directions, and that portion of it, symbolized by the wave l5 and reaching the receptor 5 along the path it, would be recorded by the apparatus I as a single incoming wave. In general, however, the structure beneath the explosion will comprise numerous superposed strata, these strata frequently being denser and having greater elasticity as they become progressively deeper. Where this is true it follows that the velocity of propagation of the wave will be greater in the'deeper strata, and hence the sphericity of the wave front will be destroyed. Refraction will take place wherever a change in density occurs, and as a result the original simple wave will be split up into a large number of wave fronts, interacting and interfering in a highly complex manner. The earth movement at the point 5 will therefore be due, not to a single arriving wave front i5, but to a plurality of arriving wave fronts, transmitted with different velocity through the different underlying media.

If the separation between the point of the impulse i and the receptor 5 be great enough, the first wave front to arrive will be that symbolized by the curve l1, which has been propagated at relatively high velocity through the deep lying stratum l9, along the path indicated by the dash line 20. Next will come the wave 2|, propagated along the path 22 through the intermediate stratum 24, while the wave II will arrive'laat. The record 9 will be of the character shown in Figure 3, whereon the instant of explosion is shown by the termination of the trace 25. By counting the number of cycles of the timing wave 26 the time of arrival of the wave l1, shownsymbolically in the figure by the envelope ll, maybe determined. In

the samemanner the time of arrival of the waves 2| and iii are shown by the envelopes 2| and I! on the record, and, theoretically, the wave propagation velocities and depth of the strata l8 and 24 may be determined therefrom by the methods well known in seismology.

In point of fact, however, the procedure is not so simple. What is desired from the record are the times of first arrival. If the waves were purely additive, so that the envelopes of the combined waves could be obtained by simple composition of the envelopes themselves, the problem would be easy. Actually, however, the incoming waves may arrive in any phase relationship. Should it happen that the first arrival of a second wave train should occur exactly in phase with the first train to arrive, that is, should the first increase of pressure due to the new wave occur at an instant when the first arrival was passing through zero with increasing pressure, the result would be a relatively sharp increase in amplitude, with a clearly defined resulting peak indicating the arrival of a new wave. If, however, the second arrival occurred exactly 180 out of phase with the first arrival, the resultant trace might show either a sudden decrease in amplitude in the same direction, an increase exactly 180 out of phase, (that is, a negative peak where a positive peak might be expected). Or there may be a. mere phase shift of the arriving wave. Any of these, other than the first condition, may be extremely difllcult to detect upon inspection and measurement of the trace, and the conditions are further complicated by the fact that there are always present additional earth movements of random character, traceable to such agencies as wind, vehicular movements on the earth surface, and other less obvious causes. It follows that could the initial transient only of the arriving wave train be recorded, with the following quasisteady state damped wave trains suppressed, the record would be much clearer and easier of interpretation, since the quasi-steady state portion of the wave conveys no information not already imparted by the initial arrival of the impulse and serves merely to mask and confuse the remainder of the record. a

Where the reflection shooting method is used, similar results are obtained, the difference being principally in the paths taken by the waves arriving at the receiver, as is illustrated in Figure 5. The apparatus is, in general the same, and is indicated in this figure by the same reference characters. The principal difference in the arrangement is that the receiver is ordinarily positioned nearer to the source of the impulse, i. e., sufliciently close so that the first wave to arrive at the receiver will be the wave 30, proceeding along the and designated by reference characters similar to those applied to the corresponding waves and distinguished by accents. Refracted or diffracted waves, not shown, may also appear upon the records made in practice.

Thus far the: procedure described does not depart materially from the prior art, the first radical departure occurring in the method of record ing the incoming waves. Even here departure is not, in theory, an absolute necessity, for the analysis of the received impulses, although much better accomplished by the electrical method later to be described, may be carried out in similar manner by a step-by-step process from the received graph as recorded by an oscillograph or equivalent apparatus. In practice, however, the time and labor saved and the accuracy achieved by the preferred process a e so much greater than those obtainable by step-oy-step procedure as to make the preferred method almost essential. Instead of recording the received waves as a trace which may be analyzed by inspection and measurement, the incoming wave is translated into a phonographic record. The particular method of phonographic recording used is not an essential of the process. The record may be made on a wax disc or cylinder, a variably magnetized wire, or may be photographically recorded, by either variable density or variable area method, on film. Any method known to the artof sound reproduction falls within the scope of this invention. For). certain practical reasons, however, I prefer to use the photographic methods of recording sound on film, with some modifications adapting the process particularly to the problem involved, as will next be described.

For reasons which will be considered in detail below, it is usually desirable to record the earths movement ensuing upon an initial impulse, at a number of spaced positions, simultaneously and in known time-phase relationship. The facility with which this may be done by the sound-onfilm systems of recording is one of the principal advantages of these systems. I prefer, therefore, to employ a plurality of microphones or pickups 50, SI, 52 and 53. each connected with its individual amplifier 55. 56, 51 and 58. The output of each of these amplifiers leads into a suitable recording lamp 60, BI, 62 and 63.

The recording lamps are positioned to illuminate an otherwise unexposed motionpicture film 65, through the recording slits 66, the film being uniformly passed relative to the slits by means of suitable driving mechanism, symbolized by the sprocket 61. Each of the recording lamps B0 to 63 inclusive is biased by means of a constant current which excites the lamp to approximately one-half normal brilliancy. As the film is passed beneath the recording slit, the output of the amplifier will modulate the light delivered by the lamp, increasing it above and decreasing it below the mean brilliancy, and exposing the film to a greater or less extent to form variable density vibration records, as designated on the drawings by the reference characters 68 to H inclusive. It will be understood that the drawings are purely symbolic, since the recorded vibration tracks are, of course, not visible upon the film until after the usual photographic development.

The lamps 60 to 63 inclusive may, of course, be glow lamps such as are customarily used in sound recording. I prefer, however, to utilize in place of these glow lamps ordinary filament type incandescent lamps of small size, and have found that the type of lamp provided for surgical use in cystoscopy are admirably suited to the purpose. The use of incandescent lamps is preferable because it is highly desirable to eliminate largely the higher frequency vibrations from the record, instead of attempting to record all frequencies equally, as is desirable in sound work. Experience has shown that the major portion of the energy transmitted through the earth as elastic waves from explosive impulses is comprised within a group of frequencies about two octaves wide with an energy maximum at about fifty cycles per second. The undifferentiated interfering waves, which have no relation to the explosion, however, carry a large portion of heir energy at much higher frequencies. By the use of a system of recording which is not responsive to these higher frequencies, these interfering waves are largely filtered out, and the system just described accomplishes this filtration without use of the auxiliary apparatus, with its accompanying.

waste and complications. The cystoscopic lamps used respond relatively slightly to the higher frequencles, their response falling gradually and substantially uniformly from a maximum at ten cycles per second and under, so that the amplitude of response at one hundred cycles will be only about half that at fifty cycles, while at five hundred cycles it is only as great. Curve 12 of Figure 15 shows the response of the lampin terms of per cent of its response at fifty cycles.

In addition to the falling frequency response characteristic of the lamps, the width of the apertures or slits 86 is preferablyso related to the speed of relative motion between the slits and the film that the movement of the film in one one hundredth of a second is equal to the width of the slit. Under these circumtsances any wave having a frequency of one hundred cycles per second or any integral multiple thereof, will fail to record altogether. Between these frequencies there will be some recordation, but, as shown by curve 13, owing to the combined effect of the aperture and the diminishing response of the lamp at the higher frequencies, the maximum response as recorded at frequencies above one hundred cycles is at most only about /10 of the fifty cycle response, while the average response of frequencies above one hundred cycles is greatly below this. This attenuation of the higher frequencies is satisfactory for all practical purposes, suppressing interfering waves to a great extent while permitting satisfactory recordation of the principal frequencies initiated by explosion. Of course it is understood that the adjustment of the apertures to a hundred cycle cut-off is not critical, but merely indicates the range in which it is preferable cut-01f should occur.

The reproducing or translating apparatus used for analysis of the records thus made also preferably differs in detail from ordinary sound reproducing equipment. It is preferable that the final analyzer trace from which the measurements and interpretations are made should show the various wave forms in ordinary Cartesian coordinates and at a sufficient amplitude so that they may readily be interpreted by inspection. It is undesirable to have the characteristics of the recording apparatus superposed upon the wave form, and since this transcribing equipment necessarily possesses some considerable inertia it is better to operate it at relatively low frequencies. 0n the other hand, amplifying equipment is easier and simpler to construct if frequencies fairly well up in the audible band be used. Both of these requirements may be met by translating the record at alower speed than that at which it was recorded, and modulating the variationsoithe vibration track upon a relatively high fremicrometer screw 73, and in azimuth. Mounted on one side of the film is a lamp which uniiormly illumines the slit, while on the other side of the film and slit is a photoelectric cell 8 i which connects to an amplifier-detector 82. A slotted cylindrical cage 83 surrounds the lamp 89, and is driven by a motor 85, the slots in the cage chopping the illumination from the lamp so that the light falling on the slit and the vibration tracks thereunder is interrupted at a relatively high frequency, e. g., one thousand cycles per second. That portion of the light which passes unabsorbed through the vibration tracks, reaches the photoelectric cell 8| and causes a current to fiow therein which is of the frequency of interruption of the light and is modulated in accordance with the density of the vibration track. This modulated current is amplified and detected by the apparatus 82, and the rectified output operates a suitable galvanorneter 86 which actuates a recording pen 8?.

The recording pen may make its trace on any suitable form of record, but I prefer to record on a paper strip 88 which is driven by the same mechanism which actuates the film drive 75, this arrangement being symbolized by the roller Q9 and belt 99.

Consider now the effect of a single one of the vibration tracks traversed across the diaphragm '16, with a reproducing slit adjusted to elementary width, i. e., to a width which is negligibly small in comparison with the length of a single cycle of a recorded earth wave of the type shown in curve A of Figure 1. That portion of the record made before the arrival of the wave train will be uniformly illuminated, and the recording pen 8? will draw a straight line on the record strip 38. When that portion of the track representing the instant of arrival of the impulse l l starts to traverse the slit, the illumination will be decreased (or increased, depending upon the phase of arrival of the wave) and the pen will swing to one side or the other in accordance with the illumination, eventually tracing out the entire wave form as shown in the graph.

If, nowrthe width of the slit be progressively increased, the effect of the first arrival will be the same, but if the increase in width of the slit be carried to the point where the width of the slit is equal to the length upon the record of one full cycle of the incoming wave, all of the damped wave train after the first arrival will tend to be suppressed by aperture effect.

This is illustrated in Figure 12. It will be seen that with the wide slit herein specified, the amount of light transmitted by the film will start to decrease at the instant when the first darkening of the film starts to traverse the slit, and

will continue progressively to decrease while that portion of the record representing the first rise of the wave from zero intensity to a maximum and its fall back to zero again are passing into the opening or the slit, i.-e., during the period or entry to the aperture of the first complete halfcycle of the incoming wave. cycle 01' the incoming wave passes into the aperture, there will be an increase in the light falling on the photoelectric cell, and this increase will almost exactly equal the preceding decrease, differing therefrom only by the damping factor, or the difl'erence in amplitude between the successive positive and negative swings of the wave. From this point forward, each change in illumination at one edge of the aperture will be almost exactly compensated by an opposite change in illumination adjacent the other edge of the aperture, and as a result the pen 8'! will come back substantially to the median line which represents the absence of an incoming wave, its excursions to either side thereof being extremely small, and depending upon the damping factor of the wave.

The resultant analyzer trace of the wave will therefore comprise a single hump, whose length corresponds substantially to a single wavelength of the incoming wave and which represents the initial transient or impulse, and the ensuing wave train will be nearly it not quite completely suppressed, as is shown in curve C of Figure 1.

In practice, the proper width of slit is found empirically. With a given series of wave trains, repeated analyzer traces may be made, with the width of the slit varied progressively from trace to trace. A width of slit will quickly be found, corresponding approximately to a record wave length representing fifty cycles per second, where only the initial impulse is recorded, and the following damped wave train is suppressed to a point where it no longer complicates the record.

.Since each of the waves whose recordation is desired originates from a single impulse, and each represents, in reality, a portion of the same initial wave, this adjustment need be made but once for any single record, and usually, in practice, but once for any group of records made in the same territory or in soil of the same character and with shots of approximately equal magnitude.

A further refinement of the method is illustrated in Figure 11. In this case a diaphragm is used comprising two mutually slidable portions 92 and 93, each carrying a recording slit 95 and 96 respectively. Relative motion of the two parts of the diaphragm, which is controlled by a micrometer screw 97, varies the separation of the reproducing slits, each of which is of elementary width. The separation of the slits is varied by trial and error, in exactly the same manner as the width of the slit in the apparatus previously described. When the separation of the slits becomes equal to one-half wave length, the translation of the record from the two separated recording points becomes exactly out of phase, and all of the waves following the first impulse cancel as before. By interposing a shutter 9% over a portion of the trailing slit, or by making this trailing slit narrower than the leading slit in the ratio of the damping factor, an exact neutralization of the successive cycles of the damped wave train may be achieved. Furthermore, in this modification of the method the initial impulse is restricted to a record wave length of onehalf cycle only, instead of a full cycle as is the case where the wider aperture is used. The resulting trace is shown by curve D of Figure 1.

It will be noted that although the procedures in the two modifications difier slightly, the princi- As the next halfpie is the same. In each case there is a multiple translation of the record, i. e., the translations of more than a single elementary area thereof, differing in time-phase, are combined, with the result that the recordation of that portion of the arriving wave train representing the initial transient impulse is accentuated at the expense of the ensuing quasi-steady state portion of the train, which is partially or completely suppressed. It is obvious that the result may be achieved in still other ways, as, for example, by changing the azimuth of the slit as related to the azimuth at which the record was made. In this manner the reproducing aperture may extend diagonally across a portion of the vibration track comprising a complete wave length, and the effect will be the same as though the aperture were one record wave length in width, although, in fact, its width is still of elementary dimension.

The advantage of this method may clearly be seen by comparing the analyzer trace shown in Figure 4 with Figure 3, the two figures representing different types of records of the same arriving wave. In Figure 4 the arriving wave trains are represented by single clearly defined and well spaced impulses, and the later arrivals are not masked and made indeterminate by the confusion due to phase diiferences between the newly arriving wave and the quasi-steady state portion of the preceding one. This method has even greater value when reflection shooting is used, as will be seen by comparison of Figures 6 and 7. Closely spaced arrivals are clearly shown, even though a later arrival may be of much less magnitude than one closely preceding it.

Figure 9 illustrates schematically the use of the method to determine the direction as well as the length of the wave paths. In this utilization the plurality of receivers 50 to 53 inclusive are positioned at substantially equal distances apart along a line passing through the point of origin of the impulse. The earth waves initiated by the impulse are reflected by an underlying stratum and reach the receiving microphones along the dashed lines terminating thereat. The separation of the various receiving microphones is ordinarily small in comparison with the distances traversed by the waves, and therefore the portion of the advancing wave front intercepted thereby may be considered as a plane. When the wave front reaches the pickup 50, its position will be approximately that shown by the dotted line I00, and no arrival will be recorded on the microphone until it has advanced the further distance indicated by the separation between the line I00 and the dotted line Illl. This additional distance of travel will be recorded on the film by a separation in time-phase between the record of arrival on vibration tracks 68 and 69, while the arrival of the wave as recorded on vibration tracks and 1| will be later by equal increments of time.

When the phonographic record is passed through the analyzer there will be components occurring in the output corresponding to eachof the separate vibration traces, the analyzer trace showing the algebraic sum of the instantaneous values of the various recordings. If the reproducing slit extends directly across the record, these arrivals will either be represented by a plurality of separate impulse traces, (Fig. 14, traces A and E) or else these impulse traces may so overlap as to form a single.hump" of longer duration (Fig. 14, traces B and D). If, now, the reproducing slit be rotated in azimuth, as indicated in Fig. 13, so that the successive arrivals on the diiferent traces all start to enter the aperture at the same instant, these separate impulse traces will be additively combined into a single pulse (Fig. 14, trace C), whose magnitude represents the sum of all of the incoming wave components. The width of the slit should, of course, be adjusted to compensate for the rotation in azimuth in the slit.

The time interval between the successive arrivals may be determined directly from the sine of the angle of rotation in azimuth of the recording slit, and from this time interval, the average speed of transmission of the wave along the reflected path being known, the effective angle of arrival of the wave front asshown by the dotted line I00 may be plotted. Furthermore, a perpendicular I02 drawn from the center of this wave front, whose length is equal to the average velocity of the wave along the reflected path times the interval between the explosion and its first arrival at the pickup microphone 50, will terminate at exposition I03, which is the apparent point of origin of the wave and which is, in fact, the virtual image of this point as reflected by the surface of the underlying layer. If, therefore, we connect the point I03 with the actual source of the explosion, I05, and erect a perpendicular I06 bisecting the connecting line, the heavy portion I01 of this perpendicular will represent the section of the surface of the underlying layer I08 from which the wave was reflected.

In practice, it is preferable to make repeated translations of the same plurality of vibration tracks, side by side on the same analyzer record strip. Each of these translations is preferably made with a slightly different azimuth of the reproducing slit, corresponding to a slightly different angle of arrival of the incoming wave front. The resulting traces representing any one arriving wave front will then, in general, be somewhat as shown in Figure 14. Where the azimuth of the slit does not correspond to the direction of the arriving wave front, there will be a partial phasing-out of the component derived from the different vibration tracks. As the azimuth of the slit approaches 'closer and closer to the proper angle of arrival, the amplitude of the analyzer trace will become successively greater, later falling in magnitude as the proper angle is passed through. This is shown in Figure 14 by the relatively large amplitude of trace C as compared with the record of the same impulses on traces A, B, D and E.

It will usually be found that different wave trains will show maximum response on different traces, the later arrivals usually, although not necessarily, reaching their maxima on traces corresponding to smaller angles of azimuth.

It will be noted, that although it is desirable to suppress all except the initial transient from each of the individual vibration tracks, this is not strictly necessary where multiple tracks are used, since many of the advantages of the method are obtained even without this feature. This is because there would usually be a difference in time of arrival of the same wave at the different receptors. It follows that the same conditions of interference will not exist in each of the vibration tracks, and that, since two wave trains will, in general, arrive from different angles, where two arrivals overlap maxima will occur at two translating azimuths, and it will therefore be pos-' sible to separate the arrivals of the two interferauger:

ing waves without first reducing them to their initial transient components.

It will be understood from the foregoing that by adding to the possible adjustments of the analyzer, much refinement of the method herein described is possible. By making the reproducing slits for the various vibration tracks individually adjustable, it is obviously possible to accommodate non-uniform placing of the pickup microphones, for the actual curvature of the advancing wave front, which therefore need not necessarily be considered as a plane surface, and for the many other conditions which may occur in practice. All such'modifications, however, fall within the general method here described, of compounding the components derived from various portions of the record in difiering time-phase relationship. The refinements relate more directly to the apparatus used, than to the general method, and are therefore discussed in detail in my copending application, Serial No. 758,834, filed December 22, 1934, Patent No. 2,051,153, dated August 18, 1936, referring specifically to the apparatus used.

Although the most generally useful method of employing my invention is that whereby the initial impulse is accentuated and the main wave train is suppressed, it will be obvious that the same system may be used for accentuating the wave train as a whole. This may be done with either a single recording, scanned a multiplicity of times by successive apertures a whole wave length apart, or by bringing into coincidence the reproductions of a wave train as received on a plurality of receptors and recorded in a manner which preserves the entire train. The latter procedure has already been touched upon, but it will be evident that the former is also useful in diiierentiating between the desired wave and chance interfering waves of different frequencies, for in general the latter will tend to phase out in an adjustment by which maximum reinforcement of the desired wave is obtained. Where a plurality of producing slits are used on a single received wave, the final combined record has a character almost precisely the same as that obtained by including in the recording circuit a resonant system tuned to the frequency of the desired wave, just as the accentuation of the first impulse is an effect which is very similar to the inclusion within the electrical circuit of a parallel resonant system tuned to the incoming wave frequency. Reinforcement by re-recording however, possesses the great advantage over the use of resonant systems of not requiring a preliminary analysis of the waves transmitted by a particular explosion and a tuning of the system to such waves -an almost impossible procedurebut of permitting the building up of the desired component of the wave under laboratory conditions where ample time is available and where the "resonant condition can be selected long after the phenomenon has passed, thus eliminating the necessity for any approximation or guess work.

The word impulse, as used in this specification, is to be understood in its broad sense of the application or effect of any impelling force, and includes the application of vibratory or quasisteady state trains as well as single individual pulses.

I claim:

1. The method of geophysical exploration which comprises the steps of initiating an earth impulse, phonographically recording earth movements ensuing upon said impulse, translating the record thus formed as a plurality of components each representative of said record and difiering from the other components in time phase as related to the initial impulse, and recording the resultant of said components 2. The method 01' geophysical exploration which comprises thesteps of initiating an earth impulse, phonographically recording earth movements ensuing upon said impulse, translating the record thus made into an electrical current comprising a plurality of, components, each representing a reproduction of said record displaced in time-phase relative to the other components, and record ng the magnitude of said current with respect to time.

3. The method of geophysical exploration which comprises the steps of initiating an earth impulse, phonographically recording the earth movements ensuing upon said impulse at a plurality of spaced positions to form multiple records thereof. and thereafter s multaneously translating said records in displaced time-phase as related to the initial impulse to form a single comrosite record representative of the algebraic sum of all of said multiple records.

4 The method of geophysical exploration which comprises the steps of initiating an earth impulse, phonogranhically recording the earth movements ensuing upon said impulse at a plurality of spaced positions. thereafter simultaneously translating the phonographic records thus formed into electrical current components, combinin said components in displaced timephase relationship with respect to the initial impulse. and recording the resultant current with respect to time.

5. The method of analysis of a complex vibration comprising a plurality of damped wave trains of random phase relationship, which comprises phonographically recording said vibra=- tion, translating simultaneously a plurality of elementary areas of the record thus formed to produce a new record representative of the algebraic sum of the movements recorded upon said areas. and varying the relationship of the areas reproduced to cause phase cancellation of the quasi-steady po tions of said damped trains, whereby the initial transients thereof become readily identifiable.

6. The method of analysis of a complex vibra tion comprising a plurality of damped wave trains of random phase relationship, which comprises phonographically recording said vibration, translating said record by taking off therfrom at a pair of spaced reproducing points, varying the spacing of said points to secure substantial phase opposition between the simultaneous translations of successive half-cycles, and re-recording the combined translations as a record showing substantially uncontaminated transients.

'7. The method of geophysical exploration which comprises the steps of initiating an earth impulse, phonographically recording the earth movements ensuing thereon at a plurality of spaced positions to form multiple simultaneous records thereof, translating said records from a plurality of reproducing points displaced in timephase with respect to said records by an amount equal to the difference in times of travel oi. earth waves from the point of origin of said impulse to the respective recording positions along different paths, and combining the translations to form a resultant record whereon the movements due to wave trains arriving by said pre .tude' as a result of phase addition and those due to wave trains arriving by other paths are-rep- "resented by lesser amplitudes due to a degree of phase cancellation. a

8. The method oi geophysical exploration which comprises the, steps of initiating an earth impulse, recording phonographically those compo- I nents oi the ensuing earth movements within a preselected frequency range at a plurality of spaced positions to form multiple simultaneous records thereof, thereafter simultaneously translating said records, phasing out oi said translations the portions representing quasi-steady state conditions, whereby the remaining transient portions are relatively accentuated, combining the resultant translations in'regularly displaced phase relationship, and re-recording the combined translations to form a single resultant trace whereon the transient impulses" resultingirom wave trains arriving from certain directions are accentuated with respect to those arriving from other directions.

9. The method of geophysical exploration which includes the steps of initiating an earth impulse, phonographically recording earth, movements ensuing upon said impulse at a plurality of spaced positions to form multiple simultaneous records thereof, repeatedly translating said records, combining. the translations, and re cording each 01' the combined translations as a visible trace, each of said translations being com- I bined in a different. time-phase relationship,

bration which comprises phonographically recording said vibration, re-translating the resingle channel all components except those having predetermined specific characteristics, and

utilizing the resultant to create a secondary record.

12. The method prises phonographically recording a plurality of records of said vibration each arriving over difierent paths, combining retranslations of said records into a single channel, filtering from said single channel all components except those having predetermined specific characteristics, and utilizing the resultant to create a secondary non phonographic record.

13. The method of analysis of a complex vibration, which comprises phonographically recording said vibration, retranslating the recorded vibration to form a secondary visual record and in the retranslatlngprocessfiltering from the translated energy substantially all components except those having predetermined specific characteristics, again retranslating therecorded vibration to form a second secondary record and in thesecond retranslation filtering from the translated energy substantially all components except those having diilerent predetermined speciiic characteristics, and comparing the records.

' FRANK RIEBER.

of analysis 0! a complex vibration set up by a single source, which com 

